A light source emits light that
passes through two narrow slits and then falls onto a screen, as shown in the
diagram. Which of the four patterns on the
screens (A), (B), (C), and (D) most correctly shows the pattern that would be
produced on the screen by the light diffracted through the slits?
To begin, let’s clear some space on
screen and recall some basic information about diffraction patterns produced by
light passing through two narrow slits.
First, we know that due to its
wavelike nature, as light passes through the slits, it interferes with itself
constructively and destructively. We can see this by placing a screen
next to the slits and noticing that the interference of the light waves results in
areas of alternating brightness and darkness on the screen. This is known as a diffraction
pattern, as the light was diffracted as it passed through and bent around the two
For light passing through two
identical slits, like in this question, the diffraction pattern is symmetric. We also expect that the brightest
area on the screen is in the center, directly between the two slits.
With all this in mind, let’s look
over the four patterns given in the diagram. Pattern (A) appears to obey the
conditions we just mentioned. The pattern of alternating
brightness and darkness is symmetric and brightest in the very center. This seems to be a good option
Next, we see that pattern (B) is
brightest in the center, but there’s no real pattern here. We expect to see alternating bright
and dark spots, not just one big bright spot, so this isn’t the best option.
Moving on, pattern (C) is
symmetrical, and we do see different areas of alternating brightness. But the very center of the screen
here is dark. We’ve already established that this
should be the brightest spot, so (C) isn’t a good option either.
Finally, pattern (D) is all over
the place. There are alternating bright and
dark spots, but they’re not symmetrical. And the very center of the screen
has a dark spot, so we know that this isn’t right either.
Thus, we’ve seen that pattern (A)
is, in fact, the best option, as it shows the properties we would expect of a
diffraction pattern produced by light passing through the two slits.